Desiccant dryers have been marketed for many years and are in wide use throughout the world. The usual type is made up of two desiccant beds, one of which is on the drying cycle while the other is being regenerated. The gas to be dried is passed through the desiccant bed in one direction during the drying cycle and then, when the desiccant has adsorbed moisture to the point that there is no assurance that the moisture level of the effluent gas will meet the requirements of the system, the gas to be dried is switched to the other bed and the spent bed is regenerated by passing purge effluent gas in a counterflow therethrough.
The purge gas may be heated before entering the bed but in many systems the bed itself is provided with heaters and the desiccant, in effect, baked out to remove the adsorbed moisture. One such system is disclosed in Seibert et al., U.S. Pat. No. 3,513,631. In the dryer of Seibert et al. there are disposed in each desiccant bed, heating elements. There is no provision, however, for detecting the failure of any heating element in either bed.
There are available a variety of sensing devices to determine when the flow of air should be switched from one desiccant bed to the other. One such device detects the moisture content in the desiccant bed and causes the drying cycle time to be modified according to the moisture load. A method of measuring the moisture load is disclosed in Seibert et al., U.S. Pat. No. 3,448,561. With moisture sensing regenerating systems of this type, the sensing probe is usually in a fixed location within the desiccant bed. If one or more of the heating elements in the desiccant bed should fail, the result of such failure will vary, depending on the location of the heating element, vis-a-vis the probe. If they are in close proximity, the probe will detect moist air and will signal that regeneration is necessary prematurely, resulting in a shorter than desired drying cycle, thus wasting energy and resulting in a shorter life for the desiccant. If the probe and heating element are not close, the probe will be unaware of the moist, undesiccated air in the area of the failed heater. Therefore, the drying will proceed as if there were no failure, resulting in moisture-laden air contaminating the effluent.
Another means for controlling the drying and regenerating cycle times is by use of a timer. Such a system is described in Christel, U.S. Pat. No. 4,322,223. A series of switches and valves are controlled by the timer to switch the heaters on and off and the gas stream from one bed to another. The time of the required cycle is determined experimentally based upon load, temperature of gas, temperature generated in the desiccant bed, effluent requirements and any other applicable factors. If the heating element should fail, the temperature reached in the desiccant bed would not be as high as expected. This could result in moisture-laden air in the effluent due to a lack of proper desiccant bed regeneration.
It is, therefore, of utmost importance to know of heating element failures in time to take the necessary steps to prevent moisture-laden air from contaminating the effluent.